The keynote is 90 minutes long. A good 30 minutes total or so is fluff, self back-patting, and demos.
That leaves 60 minutes for real announcements. I think 30 minutes for iOS, 15 minutes for Mountain Lion, and 15 minutes for something else (hardware refresh, AppleTV developer API, or something like that).
With iOS they will do the whole "tentpole" thing where they'll talk about 3-5 major areas and then say "over 200 new features we didn't talk about are in iOS 6".... 50% of which will just be new API calls for devs.
Mountain Lion will most likely be a complete rehash of what we already know with maybe a thing or two thrown in, as well as pricing and release date.
Let's keep expectations low people. Apple probably isn't going to blow anyone away (at least from people here on the forums daily).
I've learned my lesson year after year after year with over-predicting Apple events.